You could loop thru all the controls on the form and add the control's left
and width properties, which will give you the right most number of the
control (in twips).  You could store results in an array and find the
largest number, which would be the fartherest right point of a control.  I
am winging it here, don't have any code other than a

Debug.Print Me![somecontrol].Left + Me![somecontrol].Width

HTH get you started
Damon

Loop through the Controls collection, and find the largest values of Left +
Width for the form:

Dim ctlCurr As Control
Dim lngMaxSize As Long

 For Each ctlCurr In Me.Controls
   With ctlCurr
     If .Left + .Width > lngMaxSize Then
       lngMaxSize = .Left + .Width
     End If
   End With
 Next ctlCurr

 MsgBox "The form needs to be at least " & _
   lngMaxSize & " twips."